With electric tools, such as an electric drill or disc grinder, one method of reducing the size of the electric tool is to use a brushless direct current (DC) motor, which does not require electrical connection via a brush or commutator.
As illustrated in FIG. 5, a conventional brushless DC motor includes stator 401, rotor 402 axially aligned with stator 401, heat dissipating plate 403 disposed at one end, in the axial direction, of stator 401, and circuit board 404 disposed between stator 401 and heat dissipating plate 403. Stator 401 includes a coil and rotor 402 includes a permanent magnet. Heat dissipating plate 403 includes a plate-shaped main body 403a, and feet 403b. A sensor, such as a Hall element, and a switching element, such as a transistor, are consolidated on circuit board 404.
When such a brushless DC motor is rotationally driven, the rotation angle of rotor 402 relative to stator 401 is measured by the sensor, and the direction in which the field current of the coil flows is switched by the switching element in accordance with the sensor measurement. However, the high switching speed of switching element at high rotational speeds generates a large amount of heat. As a result, dissipating the heat in the switching element on circuit board 404 on which components are consolidated is important. Accordingly, circuit board 404 is attached so as to be in contact with main body 403a of heat dissipating plate 403 to dissipate heat.
Moreover, since the rotation angle of rotor 402 relative to stator 401 is measured using a sensor, the sensor needs to be fixed so as to be stationary relative to stator 401. Accordingly, heat dissipating plate 403 is fixed in place via fixing screws 406 inserted into screw holes 407 in the four corners of iron core 405 such that feet 403b do not contact iron core 405 and move. As a result of feet 403b of heat dissipating plate 403 being fixed to iron core 405, the sensor is fixed so as to be stationary relative to stator 401 via heat dissipating plate 403 and circuit board 404.